1. Field of the Invention
The present invention relates to a photomask used in a near-field exposure system for micropattern transfer. The present invention also relates to a process for producing the photomask.
2. Description of the Related Art
Since packing densities of semiconductor chips are increasing, higher resolution is required in photolithography. In response to the requirement for high resolution, exposure wavelengths have been shortened, and illumination techniques have been improved. However, when line widths in the order of 0.1 micrometers or below are required, further improvement is needed.
In the above situations, conventionally, techniques of exposure with X-rays having shorter wavelengths or electron beams are proposed. However, those techniques have drawbacks of high equipment cost and low throughput.
For example, in distributed Bragg reflector (DBR) or distributed feedback (DFB) semiconductor laser devices, gratings are formed inside the semiconductor laser devices. In such semiconductor laser devices, sometimes grating patterns having line widths in the order of 0.1 micrometers or below are required. Generally, the gratings may be realized by high-order gratings. In the case of the high-order gratings, it is easy to form a grating because the grating pitch becomes large. However, in the high-order grating, an amount of fed-back light is reduced due to spatial diffraction light, and it is necessary to control the line-and-space ratio with high accuracy. Therefore, it is preferable to realize the gratings by first-order gratings. In the case of the first-order gratings, the required dimensions of the grating patterns are in the order of 0.1 micrometers or below. Currently, the grating patterns are formed by directly writing the grating patterns with electron beams. However, according to the conventional techniques, expensive equipment is needed, and throughput is low.
Recently, the so-called near-field exposure technology is receiving attention. The near-field exposure enables transfer of micropatterns which are finer than the diffraction limit. In the near-field exposure technique, a photomask having openings which are smaller than the wavelength of exposure light is used to expose an object such as a photoresist layer to near-field light emerging from the openings of the photomask. Since the depth and extent to which the near-field light substantially propagates are smaller than the wavelength of exposure light, the near-field light enables transfer of a micropattern having dimensions smaller than the wavelength of exposure light, to the object which is to be exposed. Due to the small depth of propagation of the near-field light, the so-called contact exposure method is used.
Conventionally, the photomasks used in the contact exposure method are produced as follows.
A shading film is formed on a surface of a mask support made of a material such as glass, which is transparent to exposure light. In the shading film, an antireflection film is added to a metal film such as a chromium film. Then, the shading film is coated with a photoresist. Next, a resist pattern with openings having smaller widths than wavelength of exposure light is formed by electron beam exposure or the like. Finally, the shading film is etched by using the resist pattern as a mask so as to produce mask openings.
Alternatively, the photomasks used in the contact exposure method may be produced as explained below with reference to FIGS. 10A to 10D.
A mask support 1 made of glass or the like is provided as illustrated in FIG. 10A. Then, the surface of the mask support 1 is coated with photoresist, and a resist pattern 2 is formed by electron beam exposure or the like, as illustrated in FIG. 10B, where the line widths of the resist pattern 2 are smaller than the wavelength of the exposure light. Next, a shading film 3 is formed by sputter deposition of chromium, using the resist pattern 2 as a mask, as illustrated in FIG. 10C. Finally, the resist pattern 2 is removed so as to produce mask openings 4 at the positions from which the resist pattern 2 is removed, as illustrated in FIG. 10D.
In the conventional photomasks produced as above, a shading film is deposited on a planner surface of a mask support, and the shading film has openings which are arranged to form a predetermined pattern. In the case where such photomasks are used in the near-field exposure, exposure light is applied through the photomask to an object such as a photoresist layer, from the opposite side to the above surface on which the shading film is formed, so that near-field light emerges through the above openings of the shading film, and the object is exposed with the near-field light when the object is placed in contact with or in proximity to the shading film.
Nevertheless, since in the conventional photomasks having the above construction, the near-field light propagates to only a small distance from the mask support, it is impossible to sufficiently thicken the shading film. Therefore, the following problems arise.
The depth to which the emerged near-field light propagates, i.e., the distance to which the near-field light propagates from the surface of the mask support in the photomask having the above construction, is at most tens of nanometers. Therefore, unless the thickness of the shading film is at most tens of nanometers, the near-field light cannot reach the object which is to be exposed, even when the object is placed in contact with or in proximity to the shading film.
However, when the thickness of the shading film is sufficiently thin, i.e., at most tens of nanometers, the shading film is prone to suffer defects such as pinholes. In addition, transmittance of light through the shading film increases with decrease in the thickness of the shading film. Therefore, when the shading film is thinned, the extinction ratio, i.e., the ratio of the amount of light transmitting through the shading film and the amount of light transmitting through the openings of the shading film becomes small, and fogging may occur in the case where the exposed object is highly sensitive.
Further, in the case of contact exposure, the shading film may be damaged by the contact with the object which is to be exposed, for example, during the operation of aligning the photomask with the object. That is, the durability of the photomask is insufficient.
However, if the above shading film is thickened to exceed tens of nanometers, the near-field light cannot reach the object to be exposed at all. Even when the thickness of the shading film is tens of nanometers, the near-field light is greatly attenuated so that the photoresist cannot receive a sufficient amount of exposure light. Thus, the upper limit of the thickness of the shading film is estimated to be 50 nm.
In addition, when the aforementioned grating pattern is produced by using the conventional photomasks and directly writing a micropattern with an electron beam, only a small margin is allowed for control of the line-and-space ratio, and the production cost becomes high.
An object of the present invention is to provide a photomask which enables exposure of an object such as a photoresist layer to near-field light with a sufficient intensity, and allows formation of a sufficiently thick shading film having high durability and preventing fogging.
Another object of the present invention is to provide a process for producing a photomask which enables exposure of an object such as a photoresist layer to near-field light with a sufficient intensity, and allows formation of a sufficiently thick shading film having high durability and preventing fogging.
Still another object of the present invention is to provide a photomask which can be used in production of a grating pattern, allows a great margin for control of a line-and-space ratio of the grating pattern during the production of the photomask, and can be produced at a low cost.
A further object of the present invention is to provide a process for producing a photomask which can be used in production of a grating pattern, allows a great margin for control of a line-and-space ratio of the grating pattern during the production of the photomask, and enables production of the grating pattern at a low cost.
(1) According to the first aspect of the present invention, there is provided a photomask for use in near-field exposure, comprising a mask support which is transparent to exposure light; a shading film which is formed on one side of the mask support, and has at least one opening arranged to form a predetermined pattern; and at least one filler which is transparent to the exposure light, and is arranged in the at least one opening with a predetermined height above the level of the boundary between the mask support and the shading film.
The photomask according to the first aspect of the present invention may also have one or any possible combination of the following additional features (i) to (iii).
(i) The difference between the thickness of the shading film and the height of each of the at least one filler may be determined so as not to exceed 50 nanometers.
(ii) Each of the at least one filler may have a shape which is gradually narrowed toward the top of the filler.
(iii) Each of the at least one filler may be made of a different material from the mask support, and is coupled to the mask support.
(2) According to the second aspect of the present invention, there is provided a process for producing a photomask for use in near-field exposure, comprising the steps of: (a) forming, on one side of a flat mask support which is transparent to exposure light, a shading film having at least one opening arranged to form a predetermined pattern; and (b) forming at least one filler which is transparent to the exposure light, in the at least one opening with a predetermined height above the level of the boundary between the mask support and the shading film.
The photomask according to the first aspect of the present invention can be produced by the process according to the second aspect of the present invention.
The process according to the second aspect of the present invention may also have one or any possible combination of the following additional features (iv) to (vi).
(iv) The at least one filler may be formed by embedding, in the at least one opening, a different material from the mask support.
(v) The process according to the second aspect of the present invention may further comprise the step (c) of planarizing one side of the mask support, after the shading film and the filler are formed on the mask support in the steps (a) and (b), by a polishing operation, so that the difference between the thickness of the shading film and the height of each of the at least one filler does not exceed 50 nanometers.
(vi) The process according to the second aspect of the present invention may further comprise the step (cxe2x80x2) of planarizing one side of the mask support, after the shading film and the filler are formed on the mask support in the steps (a) and (b), by an etching-back operation, so that the difference between the thickness of the shading film and the height of each of the at least one fillers does not exceed 50 nanometers.
(3) According to the third aspect of the present invention, there is provided a photomask for use in near-field exposure, comprising: a mask support which is transparent to exposure light; and a shading film which is formed on one side of the mask support, and has at least one opening arranged to form a predetermined pattern. The mask support comprises at least one protrusion so as to fill the at least one opening, and have a predetermined height above the level of the bottom of the shading film.
The photomask according to the third aspect of the present invention may also have one or any possible combination of the following additional features (vii) and (viii).
(vii) The difference between the thickness of the shading film and the height of each of the at least one protrusion may be determined not to exceed 50 nanometers.
(viii) Each of the at least one protrusion may have a shape which is gradually narrowed toward the top of the protrusion.
(4) According to the fourth aspect of the present invention, there is provided a process for producing a photomask for use in near-field exposure, comprising the steps of: (a) etching a surface, other than at least one portion corresponding to at least one opening of a shading film, of a flat mask support which is transparent to exposure light, to a predetermined depth, so as to form at least one concave portion, and leave at least one protrusion corresponding to the at least one opening, where the at least one protrusion has at least a predetermined height above the level of the bottom of the at least one concave portion; and (b) forming the shading film in the at least one concave portion.
The photomask according to the third aspect of the present invention can be produced by the process according to the fourth aspect of the present invention.
The process according to the fourth aspect of the present invention may also have one or any possible combination of the following additional features (ix) and (x).
(ix) The process according to the fourth aspect of the present invention may further comprise the step (c) of planarizing one side of the mask support, after the shading film is formed on the mask support in the step (b), by a polishing operation, so that the difference between the thickness of the shading film and the height of each of the at least one protrusion does not exceed 50 nanometers.
(x) The process according to the fourth aspect of the present invention may further comprise the step (cxe2x80x2) of planarizing one side of the mask support, after the shading film is formed on the mask support in the step (b), by an etching-back operation, so that the difference between the thickness of the shading film and the height of each of the at least one protrusions does not exceed 50 nanometers.
(5) According to the fifth aspect of the present invention, there is provided a photomask for use in near-field exposure, comprising: a mask support which is transparent to exposure light, and has a plurality of concave portions on one side of the mask support so as to form at least one convex portion between the plurality of concave portions; and a shading film which is embedded in the plurality of concave portions, and has at least one opening arranged to form a predetermined pattern, where the at least one convex portion corresponds to the at least one opening. In the photomask, the difference between heights of the top surfaces of the at least one convex portion and the shading film does not exceed 50 nanometers.
The photomask according to the fifth aspect of the present invention may also have the following additional feature (xi).
(xi) Each of the at least one convex portion may have a shape which is gradually narrowed toward the top surface of the convex portion.
(6) According to the sixth aspect of the present invention, there is provided a process for producing a photomask for use in near-field exposure, comprising the steps of: (a) producing a mold having a plurality of first convex portions and at least one first concave portion, where the plurality of first convex portions are provided for forming a plurality of second concave portions on one side of a mask support, and the at least one first concave portion is provided for forming at least one second convex portion on the side of the mask support; (b) forming the mask support by using the mold, where the mask support is transparent to exposure light, and has the plurality of second concave portions on the side of the mask support and the at least one second convex portion between the plurality of second concave portions; (c) depositing a shading material on an entire area of the mask support so as to fill the plurality of second concave portions with the shading material, and form a layer of the shading material; and (d) removing a portion of the layer of the shading material by performing a planarizing operation on the layer of the shading material so as to expose at least a top surface of each of the at least one second convex portion, and form a shading film with a remaining portion of the layer of the shading material.
The photomask according to the fifth aspect of the present invention can be produced by the process according to the sixth aspect of the present invention.
The process according to the sixth aspect of the present invention may also have one or any possible combination of the following additional features (xii) to (xv).
(xii) In the step (a), the mold may be made of silicon, and formed by anisotropic etching.
(xiii) In the step (a), the at least one first concave portion may have a shape which is narrowed toward a top of each of the at least one first concave portion.
(xiv) In the step (d), the planarizing operation may be realized by a polishing operation.
(xv) In the step (d), the planarizing operation may be realized by an etching-back operation.
(7) The advantages of the photomasks according to the first and third aspects of the present invention are explained below.
In the photomask according to the first or third aspect of the present invention, a filler or protrusion of the mask support is arranged in each opening of the shading film, and the filler or protrusion has a predetermined height above the level of the boundary between the mask support and the shading film. That is, even when the thickness of the shading film is increased, the filler or protrusion can be formed to have a sufficient height so that near-field light emerging from the surface of the filler or protrusion can reach an object which is to be exposed, immediately or within a very short distance, i.e., before the near-field light is greatly attenuated. Thus, it is possible to sufficiently increase the thickness of the shading film so as to secure a sufficient extinction ratio, prevent the aforementioned fogging, and enhance the durability of the shading film.
The top surface of the filler or protrusion and the top surface of the shading film may be located at the same distance from the object, or the top surface of the filler or protrusion may be located nearer to or farther from the object, than the top surface of the shading film. In particular, it is preferable that there is a relationship between the thickness d of the shading film and the height h of the filler or protrusion as follows.
hxe2x88x9250(nm)xe2x89xa6dxe2x89xa6h+50(nm)
That is, the difference between the thickness d of the shading film and the height h of each of the at least one filler may be determined so as not to exceed 50 nanometers.
The advantages of the photomasks according to the first and third aspects of the present invention are explained for the three cases of h=d, h less than d, and h greater than d.
(a) First, the advantages in the case where the top surface of the shading film is flush with the top surface of the filler or protrusion, i.e., h=d, are explained below.
In this case, when the photomask is placed so that the shading film is in contact with an object such as a photoresist layer for near-field exposure, the top surface of the filler or protrusion is also in contact with the object. Therefore, near-field light emerged from the top surface of the filler or protrusion can enter the object immediately. Thus, the object can be exposed to the near-field light with a sufficient intensity.
When the photomask is placed at a small distance, not exceeding 50 nanometers, from an object such as a photoresist layer during near-field exposure, the propagation distance of the near-field light from the top surface of the filler or protrusion to the object does not exceed 50 nanometers. Therefore, the near-field light emerging from the top surface of the filler or protrusion can reach the object before the near-field light is greatly attenuated. Therefore, the object can also be exposed to the near-field light with a sufficient intensity.
In addition, the thickness d of the shading film can be increased to a necessary amount, as far as the height h of the filler or protrusion above the level of the boundary between the mask support and the shading film can be increased with the thickness d of the shading film. That is, the thickness of the shading film can be increased sufficiently so as to secure a sufficient extinction ratio, prevent the aforementioned fogging, and enhance the durability of the shading film.
(b) Next, the advantages in the case where the height h of the filler or protrusion is smaller than the thickness d of the shading film, i.e., h less than dxe2x89xa6h+50(nm), are explained below.
In this case, when the photomask is placed so that the shading film is in contact with an object such as a photoresist layer for near-field exposure, the top surface of the filler or protrusion is at a very short distance from the object. Therefore, near-field light emerging from the top surface of the filler or protrusion can reach the object before the near-field light is greatly attenuated. Therefore, the object can also be exposed to the near-field light with a sufficient intensity.
When the difference between the height h of the filler or protrusion and the thickness d of the shading film is very small, and during near-field exposure the shading film is located at such a short distance from the object that the propagation distance of the near-field light from the top surface of the filler or protrusion to the object does not exceed 50 nanometers, and the near-field light emerging from the top surface of the filler or protrusion can reach the object before the near-field light is greatly attenuated. Therefore, the object can also be exposed to the near-field light with a sufficient intensity.
In addition, the thickness d of the shading film can be increased to a necessary amount, as far as the height h of the filler or protrusion above the level of the boundary between the mask support and the shading film can be increased so that the difference between the height h of the filler or protrusion and the thickness d of the shading film does not exceed 50 nanometers. Therefore, the thickness of the shading film can be increased sufficiently so as to secure a sufficient extinction ratio, prevent the aforementioned fogging, and enhance the durability of the shading film.
(c) The advantages in the case where the height h of the filler or protrusion is greater than the thickness d of the shading film, i.e., hxe2x88x9250(nm)xe2x89xa6d less than h, are explained below.
In this case, when the photomask is placed in contact with an object such as a photoresist layer for near-field exposure, the top surface of the filler or protrusion is in contact with the object. Therefore, near-field light emerging from the top surface of the filler or protrusion can immediately enter the object. Therefore, the object can also be exposed to the near-field light with a sufficient intensity.
Even when the photomask is placed at a short distance from an object such as a photoresist layer during near-field exposure, the near-field light emerged from the top surface of the filler or protrusion can reach the object before the near-field light is greatly attenuated, as far as the propagation distance of the near-field light from the top surface of the filler or protrusion to the object does not exceed 50 nanometers. Therefore, the object can also be exposed to the near-field light with a sufficient intensity.
In addition, the thickness d of the shading film can be increased to a necessary amount, as far as the height h of the filler or protrusion above the level of the boundary between the mask support and the shading film can be increased with the thickness d of the shading film. That is, the thickness of the shading film can be increased sufficiently so as to secure a sufficient extinction ratio, prevent the aforementioned fogging, and enhance the durability of the shading film.
Further, the height h of the filler or protrusion is limited so that the difference between the height h of the filler or protrusion and the thickness d of the shading film does not exceed 50 nanometers. This is because the resolution of an image generated by exposure may be lowered due to spread of the near-field light when the height h of the filler or protrusion exceeds the thickness d of the shading film by too much.
(8) The advantages of the photomask according to the fifth aspect of the present invention and the process for producing a photomask according to the sixth aspect of the present invention are explained below.
In the photomask according to the fifth aspect of the present invention, the mask support has at least one convex portion and a plurality of concave portions, a shading film is embedded in the plurality of concave portions, and the difference between heights of the top surfaces of the at least one convex portion and the shading film does not exceed 50 nanometers. The top surfaces of the at least one convex portion and the shading film may be located at the same distance from the object.
Alternatively, the at least one top surface of the at least one convex portion may be located nearer to or farther from the object, than the top surface of the shading film. The relationship between the thickness d of the shading film and the height h of the filler or protrusion can be expressed as follows.
hxe2x88x9250(nm)xe2x89xa6dxe2x89xa6h+50(nm)
Since the shading film is embedded in the plurality of concave portions of the mask support, the thickness of the shading film can be increased to a necessary amount by increasing the depth of the plurality of concave portions of the mask support. Therefore, the thickness of the shading film can be increased sufficiently so as to secure a sufficient extinction ratio, prevent the aforementioned fogging, and enhance the durability of the shading film.
When each of the at least one convex portions has a shape which is gradually narrowed toward the top surface of the convex portion, the attenuation of the exposure light during the propagation through each convex portion can be reduced. Therefore, the object can be efficiently exposed to the near-field light.
Next, the advantages of the process for producing a photomask according to the sixth aspect of the present invention are explained below.
The mask support including the at least one second convex portion and the plurality of second concave portions can be produced by using a mold having the plurality of first convex portions and the at least one second concave portion. Therefore, once a mold is produced, a number of identical mask supports can be produced easily. Thereafter, a number of photomasks can be produced by operations such as evaporation, polishing, etching, and the like, which can be performed on a plurality of mask supports by batch processing.
In particular, when a mold is produced from a silicon wafer, and the convex and concave portions are formed by anisotropic etching, accurate processing is possible.
When each of the at least one first concave portions of the mold has a shape which is gradually narrowed toward the bottom thereof, each of the at least one second convex portions of the mask support has a shape which is gradually narrowed toward the top surface of the convex portion. Therefore, the width of the top surface of each of the at least one convex portions of the mask support can be accurately adjusted by controlling the amount of the shading material removed by the planarization processing, e.g., by controlling the etched depth when the planarization is realized by etching.
The width of the top surface of each of the at least one convex portions of the mask support is the width of each opening of the shading film. Therefore, when a grating pattern is produced by near-field exposure using a photomask produced by the process according to the sixth aspect of the present invention, the line-and-space ratio of the grating pattern can be accurately adjusted by controlling the amount of the shading material removed by the planarization processing. Namely, even when the mold is produced with a relatively large margin, a desired line-and-space ratio of the grating pattern can be obtained by appropriately controlling the amount (depth) of the removed shading material. When large margins are allowed for the dimensions of the convex and concave portions of the mold, the mold can be manufactured in a short time at low cost. Therefore, the photomask produced by the process according to the sixth aspect of the present invention can be manufactured in a short time at low cost.